1. Field of the Invention
This invention relates to an image display apparatus and more particularly to an image display apparatus of the active matrix type wherein electro-optical elements such as organic EL (Electro Luminescence) light emitting elements are arrayed in a matrix. The present invention relates more specifically to a screen luminance adjustment technique for a self-luminous image display apparatus.
2. Description of the Related Art
An image display apparatus of the active matrix type wherein a light emitting element such as an organic EL element is used for a pixel is known in the past and disclosed, for example, in Japanese Patent Laid-Open No. 2001-60076. The existing image display apparatus basically includes scanning lines, signal lines, and pixel circuits in order to carry out line-sequential scanning over one field. The scanning lines extend along the direction of a row and successively supply a control signal in synchronism with horizontal periods. The signal lines extend along the direction of a column and supply an image signal in accordance with the line-sequential scanning. The pixel circuits are disposed at intersecting locations of the scanning lines and the signal lines and form pixels. Each of the pixel circuits includes at least a sampling transistor, a driving transistor, a switching transistor, and an electro-optical element such as an organic EL light emitting element. The sampling transistor is rendered conducting in accordance with a control signal supplied from the associated scanning line in accordance with one horizontal period to sample an image signal supplied thereto the associated signal line. The driving transistor supplies output current in response to the sampled image signal to the electro-optical element. The electro-optical element emits light at a luminance based on the image signal with output current supplied from the driving transistor to display an image on the screen. The switching transistor is disposed on a current path along which the output current flows and carries out on and off operations in response to a different control signal supplied thereto from the scanning line. When the switching transistor is in the off state, it interrupts output current thereof, but when the switching transistor is in the on state, it supplies output current to the electro-optical element to cause the electro-optical element to emit light. The light emitting period within which any of the electro-optical elements emits light is controlled within one field in this manner to adjust the luminance level (peak luminance) of the screen.
By variably adjusting the light emitting period in this manner, the peak luminance of the screen can be controlled without varying the amplitude of the input image signal. As the light emitting period per one field increases, the light emitting amount per one field increases as much and the luminance of the screen perceived by a human being increases as much. Conversely, as the light emitting period per one field decreases, the light emitting amount per one field decreases as much and the luminance of the screen perceived by a human being decreases as much. This signifies that, where the input image signal is a digital signal, the peak luminance can be controlled without decreasing the number of gradations of the signal. Meanwhile, where the input image signal is an analog signal, since the signal amplitude does not decrease, the withstanding property against noise is high. Consequently, an image display apparatus is implemented which achieves high picture quality and allows peak luminance control.